Some times ago, I found for me a solution, how to handle the unstable readings from the ADC. I've described it in this post.

In the meantime, some HAMs also build the RF powermeter based on the work of Makis, SV1AFN and myself. One if these HAMs, PA3AXA Rens, digged a little bit deeper and analysed the problem in detail. Here his description.

The Nextion display needs the content defined in the file PowerMeter.HMI. You can either upload it with the Nextion editor via a serial link to the display or you can copy the TFT file to an SD card and put this into the socket on the Nextion display.

I've used one of these cheap switch-mode DC-DC converters (available on eBay for a few bucks) for my WSPR U3S transmitter. Checking the transmitted signal reveals strong spurious emissions. Hints from the QRPLabs group leads me to the cause - the DC-DC converter. Replacing it with a linear LM317 one solves the problem.

I've read the PA bias study from Hans and tried to tweak the PA of my U3S for optimum harmonics suppression etc. The spectrum is measured using an SIGLENT SSA3021X analyzer. The power is measured using a 40dB tap and a cheap 20w Dummyload from Diamond: The tap is based on the work of OZ2CPU.